10/26/2017 CHAPTER 3 & 4 Stacks & Queues The Collection Framework 1
10/26/2017 Stack Abstract Data Type A stack is one of the most commonly used data structures in computer science A stack can be compared to a Pez dispenser Only the top item can be accessed You can extract only one item at a time The top element in the stack is the last added to the stack (most recently) The stack’s storage policy is Last-In, First- Out , or LIFO Java Collections: Stack The Java API includes a Stack class as part of the package java.util : Stack<String> myStringStack = new Stack<String>(); Stack<Place> myPlacesStack = new Stack<Places>(); myStringStack.push(“Deepak”); myPlacesStack.push(new Place(“19010”, “Bryn Mawr”, “PA”)); etc. 2
10/26/2017 Specification of the Stack Abstract Data Type Only the top element of a stack is visible; therefore the number of operations performed by a stack are few We need the ability to test for an empty stack (empty) inspect the top element (peek) retrieve the top element (pop) put a new element on the stack (push) A Stack of Strings “Rich” is the oldest element on the stack and “Jonathan” is the youngest (Figure a) String last = names.peek(); stores a reference to “Jonathan” in last String temp = names.pop(); removes “Jonathan” and stores a reference to it in temp (Figure b) names. push(“Philip”) ; pushes “Philip” onto the stack (Figure c) 3
10/26/2017 Other examples of stacks 7 Back button in browser Palindrome checker Go hang a salami, I’m a lasagna hog! Matching parentheses Expression evaluation printStackTrace() Queue The queue, like the stack, is a widely used data structure A queue differs from a stack in one important way A stack is LIFO list – Last-In, First-Out while a queue is FIFO list, First-In, First-Out 4
10/26/2017 Queue Abstract Data Type A queue can be visualized as a line of customers waiting for service The next person to be served is the one who has waited the longest New elements are placed at the end of the line Print Queue Operating systems use queues to keep track of tasks waiting for a scarce resource ensure that the tasks are carried out in the order they were generated Print queue: printing is much slower than the process of selecting pages to print, so a queue is used 5
10/26/2017 Specification for a Queue Interface The Queue interface implements the Collection interface (and therefore the Iterable interface), so a full implementation of Queue must implement all required methods of Collection (and the Iterable interface) Class LinkedList Implements the Queue Interface The LinkedList class provides methods for inserting and removing elements at either end of a double-linked list, which means all Queue methods can be implemented easily The Java 5.0 LinkedList class implements the Queue interface Queue<String> names = new LinkedList<String>(); creates a new Queue reference, names , that stores references to String objects The actual object referenced by names is of type LinkedList<String> , but because names is a type Queue<String> reference, you can apply only the Queue methods to it 6
10/26/2017 The Collection Framework Java Collections: Queue The Java API includes a Queue interface as part of the package java.util : Queue<String> myStringQueue = new LinkedList<String>(); Queue<Place> myPlacesQueue = new LinkedList<Places>(); myStringQueue.offer(“Deepak”); myPlacesQueue.offer(new Place(“19010”, “Bryn Mawr”, “PA”)); etc. 7
10/26/2017 Examples of Queues 15 Simulations of real life situations: Service Queues Scheduling processes in Operating Systems Keep track of state in systematic searches Stacks & Queues 16 java.util.Stack<E> java.util.Queue<E> boolean empty() boolean add(e) E peek() boolean offer(e) E pop() Both raise E remove() EmptyStackException E poll() E push(e) + all List<E> operations E peek() E element() Return T/F/null - Raise NoSuchElementException 8
10/26/2017 Stack Applications Section 3.2 Finding Palindromes Palindrome: a string that reads identically in either direction, letter by letter (ignoring case) kayak "I saw I was I" “Able was I ere I saw Elba” "Level madam level" Problem: Write a program that reads a string and determines whether it is a palindrome 9
10/26/2017 Finding Palindromes (cont.) Finding Palindromes (cont.) import java.util.*; public class PalindromeFinder { private String inputString; private Stack<Character> charStack = new Stack<Character>(); public PalindromeFinder(String str) { inputString = str; fillStack(); // fills the stack with the characters in inputString } ... 10
10/26/2017 Finding Palindromes (cont.) Solving using a stack: Push each string character, from left to right, onto a stack a k a y k k k a y a k a y a k k y a a k a y private void fillStack() { for(int i = 0; i < inputString.length(); i++) { a k charStack.push(inputString.charAt(i)); } } k Finding Palindromes (cont.) Solving using a stack: Pop each character off the stack, appending each to the StringBuilder result a k a k a k y k y a y a k y k a y a k a k a k private String buildReverse(){ StringBuilder result = new StringBuilder(); while(!charStack.empty()) { k result.append(charStack.pop()); } return result.toString(); } 11
10/26/2017 Finding Palindromes (cont.) ... public boolean isPalindrome() { return inputString.equalsIgnoreCase(buildReverse()); } } Queue Applications Discrete Event Simulation 12
10/26/2017 Discrete Event Simulation 25 Single Queue, single server Rear Front Single Queue, multiple servers Rear Front … Multiple Queue, multiple servers Rear Front … … Rear Front Example: Single Queue, Single Front Server 26 Arrival process How customers arrive: What is inter-arrival time? E.g. between 1-3 min Service mechanism: How long will service take? E.g. 0.5 to 2.0 min Queue characteristics: FIFO Rear 13
10/26/2017 Example Data 27 Customer Inter-arrival Time Service Time (min) C1 1.9 1.7 C2 1.3 1.8 C3 1.1 1.5 C4 1.0 0.9 Queue Simulation T Arrival Queue Server Depart 0 [] Idle 1.9 C1 [] C1 3.2 C2 [C2] C1 3.6 [] C2 C1 4.3 C3 [C3] C2 5.3 C4 [C4, C3] C2 5.4 [C4] C3 C2 6.9 [] C4 C2 7.8 [] C4 Application: Lab Printer Simulation 28 There is one printer in the Computer Science Lab At any given time, there may be as many as 10 students working in the lab Each student may print upto twice in an hour Print jobs are 1-20 pages long ∴ There are up to 20 print jobs in an hour Question: What is the chance that in any given second there will be a print job scheduled? 14
10/26/2017 Application: Lab Printer Simulation 29 There is one printer in the Computer Science Lab At any given time, there may be as many as 10 students working in the lab Each student may print upto twice in an hour Print jobs are 1-20 pages long ∴ There are up to 20 print jobs in an hour Question: What is the chance that in any given second there will be a print job scheduled? Application: Lab Printer Simulation 30 There is one printer in the CS Lab (10 ppm) At any given time, there may be as many as 10 students working in the lab Each student may print upto twice in an hour Print jobs are 1-20 pages long ∴ There are up to 20 print jobs in an hour Question: What will the average wait time be for students to receive their printouts? Question: What would the average wait time be if the printer were upgraded to 20 ppp? 15
10/26/2017 Implementing a Stack Section 3.3 Java Collections: Stack The Java API includes a Stack class as part of the package java.util : Stack<String> myStringStack = new Stack<String>(); Stack<Place> myPlacesStack = new Stack<Places>(); myStringStack.push(“Deepak”); myPlacesStack.push(new Place(“19010”, “Bryn Mawr”, “PA”)); etc. 16
10/26/2017 Implementing a Stack with a List Component We can write a class, ListStack , that has a List component (in the example below, theData ) We can use either the ArrayList , or the LinkedList classes, as all implement the List interface. The push method, for example, can be coded as public E push(E obj) { theData.add(obj); return obj; } A class which adapts methods of another class by giving different names to essentially the same methods ( push instead of add ) is called an adapter class Writing methods in this way is called method delegation Implementing a Stack Using an Array If we implement a stack as an array, we would need . . . Allocate storage for an array with a default capacity public class ArrayStack<E> implements StackInt<E> { Keep track of the top of the private E[] theData; stack (subscript of the element int topOfStack = -1; at the top of the stack; for private static final int INITIAL_CAPACITY = 10; empty stack = -1) public ArrayStack() { theData = (E[])new Object[INITIAL_CAPACITY]; } // ArrayStack() There is no size variable or method 17
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